NOTE-2-6-GUARDS

When you declare that a function treats certain formals
as :stobjs, the guard of the function is automatically
extended to include the corresponding stobj-recognizer calls. For example,
if a definition includes (declare (xargs :stobjs (ST))) then the
guard of the function is changed by the addition of the conjunct
(ST-P ST).

One impact of this is that if you use the built-in ACL2 state
as a formal parameter of a function, (STATE-P STATE) is added to
the guard. This may introduce a guard where there was none in
previous versions of the system. In older versions, therefore, no
attempt would be made to verify-guards, while in the new
version, we would attempt guard verification. You may wish to add
(declare (xargs :verify-guards nil)) to such definitions.

A related change affects users who do not use stobjs or state.
In previous versions of the system -- as now -- a type
declaration extended the guard you provided explicitly. Thus, if
you wrote (declare (type integer n)) then (INTEGERP n) was
added to your guard. This is still the case and :stobjs
recognizers are similarly added. But in older versions of the system
we ``added'' the conjuncts without checking whether they were already
present in the guard you provided. This sometimes produced such
guards as (and (integerp n) (integerp n)) where the first was
produced by your type declaration and the second was your
:guard. We now eliminate redundant conjuncts; this may rearrange
the order of the conjuncts.

The guard conjectures for functions using stobjs have been simplified
somewhat by taking advantage of the syntactic restrictions checked for
single-threaded objects.

The following functions have been modified so that character and
string arguments are restricted to standard characters.
(See standard-char-p and see standard-char-listp.)

Also, function standard-string-alistp replaces function
string-alistp, with concomitant changes in the guard to
assoc-string-equal, and in variable *acl2-exports*.
Also, lemma standard-string-alistp-forward-to-alistp replaces
lemma string-alistp-forward-to-alistp. There is a new lemma
standard-char-p-nth, which has also been added to *acl2-exports*.

The guard had been inadvertently omitted from the definition of the
function substitute (and its subroutine substitute-ac). This
omission has been corrected; also, the guard is slightly stronger
than the documentation had claimed (and that has been corrected).